Field of the Invention
The present invention relates to methods for manufacturing electrodes for applications such as nonaqueous electrolyte secondary batteries and methods for manufacturing secondary batteries.
Description of Related Art
Secondary batteries, such as nonaqueous electrolyte secondary batteries, have been used as power supplies for driving electric vehicles (EVs), hybrid electric vehicles (HEVs and PHEVs), and other systems.
Secondary batteries include positive and negative electrodes, each composed of a substrate made of metal foil and an active material layer containing an active material on the substrate. There is a need for a secondary battery with a higher volume energy density for electric vehicles (EVs), hybrid electric vehicles (HEVs and PHEVs), and other systems. One approach to increase the volume energy density of secondary batteries is to increase the packing density of the active material layer. This increases the amount of active material present in the casing and thus increases the volume energy density. The packing density of the active material layer can be increased, for example, by forming the active material layer on the substrate and then compressing the active material layer at a higher pressure using a machine such as a roller press.
However, if the active material layer formed on the substrate is compressed at a higher pressure, both the active material layer and the region of the substrate where the active material layer is formed are compressed at a higher pressure; therefore, the substrate is rolled. If the substrate has exposed regions where no active material layer is formed along the lateral edges of the electrode in the longitudinal direction, the exposed regions of the substrate are not pressed during compression since they are thinner than the region where the active material layer is formed. Thus, whereas the region of the substrate where the active material layer is formed is rolled during compression, the exposed regions of the substrate are not rolled. This results in a difference in length in the longitudinal direction between the region of the substrate where the active material layer is formed and the exposed regions of the substrate. The difference in length in the longitudinal direction over the substrate causes creases in the substrate and warpage in the electrode.
To solve this problem, Japanese Patent No. 5390721 (Patent Document 1) proposes a technique involving stretching exposed regions of a substrate of an electrode in advance before roller-pressing the electrode.